Signaling design to support in-device coexistence interference avoidance

ABSTRACT

A method of implicit signaling to support In-Device coexistence interference avoidance is provided. A UE sends an IDC interference indication to an eNB. The indication indicates that a serving frequency becomes unusable due to a coexistence interference problem. The indication does not explicitly indicate a frequency index or a frequency location of the unusable serving frequency. The eNB determines the serving frequency as unusable in an implicit manner. The eNB also determines an implied unusable frequency region based on the received IDC indication. The implied unusable frequency region is between the serving frequency and the ISM band. In one advantageous aspect, the eNB configures a condition for the UE, such that the UE is refrained from sending IDC interference indications unless the condition is satisfied.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 from U.S.Provisional Application No. 61/470,711, entitled “Signaling Design toSupport In-Device Coexistence Interference Avoidance,” filed on Apr. 1,2011, the subject matter of which is incorporated herein by reference.

TECHNICAL FIELD

The disclosed embodiments relate generally to wireless networkcommunications, and, more particularly, to in-device coexistenceinterference avoidance.

BACKGROUND

Ubiquitous network access has been almost realized today. From networkinfrastructure point of view, different networks belong to differentlayers (e.g., distribution layer, cellular layer, hot spot layer,personal network layer, and fixed/wired layer) that provide differentlevels of coverage and connectivity to users. Because the coverage of aspecific network may not be available everywhere, and because differentnetworks may be optimized for different services, it is thus desirablethat user devices support multiple radio access networks on the samedevice platform. As the demand for wireless communication continues toincrease, wireless communication devices such as cellular telephones,personal digital assistants (PDAs), smart handheld devices, laptopcomputers, tablet computers, etc., are increasingly being equipped withmultiple radio transceivers. A multiple radio terminal (MRT) maysimultaneously include a Long-Term Evolution (LTE) or LTE-Advanced(LTE-A) radio, a Wireless Local Area Network (WLAN, e.g., WiFi) accessradio, a Bluetooth (BT) radio, and a Global Navigation Satellite System(GNSS) radio.

Due to spectrum regulation, different technologies may operate inoverlapping or adjacent radio spectrums. For example, LTE/LTE-A TDD modeoften operates at 2.3-2.4 GHz, WiFi often operates at 2.400-2.483.5 GHz,and BT often operates at 2.402-2.480 GHz. Simultaneous operation ofmultiple radios co-located on the same physical device, therefore, cansuffer significant degradation including significant coexistenceinterference between them because of the overlapping or adjacent radiospectrums. Due to physical proximity and radio power leakage, when thetransmission of data for a first radio transceiver overlaps with thereception of data for a second radio transceiver in time domain, thesecond radio transceiver reception can suffer due to interference fromthe first radio transceiver transmission. Likewise, data transmission ofthe second radio transceiver can interfere with data reception of thefirst radio transceiver.

Various in-device coexistence (IDC) interference mitigation solutionshave been proposed. For example, an UE may request network assistance tomitigate IDC interference via frequency division multiplexing (FDM),time division multiplexing (TDM), and/or power management principles.While the various FDM, TDM, and power management solutions may solvesome of the IDC interference problems, there are still remaining issues.For example, the UE will report unusable frequencies to eNB for FDMsolutions. However, it is not yet clear that how the unusablefrequencies are judged and in which format it will be reported to eNB.It is also not clear that how the unusable frequencies are reported. Canthe report of unusable frequencies be used along with reactive orproactive trigger? Can the report of unusable frequency resolveping-pong effect? In addition, how does the UE detect IDC interferenceproblem, and can such detection be managed by eNB? A systematic approachis necessary to resolve above problems together.

SUMMARY

A method of implicit signaling to support In-Device coexistenceinterference avoidance is provided. A UE sends an IDC interferenceindication to an eNB. The indication indicates that a serving frequencybecomes unusable due to a coexistence interference problem. Theindication does not explicitly indicate a frequency index or a frequencylocation of the unusable serving frequency. The eNB determines theserving frequency as unusable in an implicit manner. The eNB alsodetermines an implied unusable frequency region based on the receivedIDC indication. The implied unusable frequency region is between theserving frequency and the ISM band. Based on the knowledge of unusablefrequency and implied unusable frequency region, the eNB can makedecision for coexistence interference avoidance without high signalingoverhead.

In one embodiment, the UE sends an IDC interference relief indication tothe eNB. The relief indication indicates that the serving frequencybecomes usable. The relief indication does not explicitly indicate afrequency index or a frequency location of the usable serving frequency.The eNB will treat the serving frequency as usable in implicit mannerafter receiving the IDC relief indication. The eNB will also treat thefrequency channels that are farther away from the ISM band than theserving frequency as usable frequency channels.

In one advantageous aspect, the eNB configures a condition for the UE,such that the UE is refrained from sending IDC interference indicationsunless the condition is satisfied to protect the network from frequentand unreliable IDC problem reporting. In a first example, the conditionis satisfied if a prohibit timer expires. In a second example, thecondition is satisfied if a Physical Downlink Control Channel (PDCCH)decoding rate is below a threshold value. In a third example, thecondition is satisfied if a downlink HARQ error rate is above athreshold value. In a fourth example, the condition is satisfied if theeNB informs the UE that the eNB is capable of supporting in-devicecoexistence interference avoidance feature.

Other embodiments and advantages are described in the detaileddescription below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 illustrates a user equipment having multiple radio transceiversin a wireless communication system in accordance with one novel aspect.

FIG. 2 is a simplified block diagram of an eNB and a user equipment inaccordance with one novel aspect.

FIG. 3 illustrates an ISM radio signal and implicitly indicatingunusable serving frequency in accordance with one novel aspect.

FIG. 4 illustrates a method of implicit signaling design for IDCinterference avoidance in accordance with one novel aspect.

FIG. 5 illustrates a method of configuring a measurement pattern inaccordance with one novel aspect.

FIG. 6 illustrates a method of providing a prohibit timer in accordancewith one novel aspect.

FIG. 7 is a flow chart of a method of IDC interference avoidance fromeNB perspective.

FIG. 8 is a flow chart of a method of IDC interference avoidance from UEperspective.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 illustrates a user equipment (UE) having multiple radiotransceivers in a wireless communication system 100 in accordance withone novel aspect. Wireless communication system 100 comprises an LTEbase station eNB 101, a WiFi access point AP 102, and a user equipmentUE 103. Wireless communication system 100 provides various networkaccess services for UE 103 via different radio access technologies. Forexample, eNB 101 provides cellular radio network (e.g., a 3GPP Long-TermEvolution (LTE) system) access, while AP 102 provides local coverage forWireless Local Area Network (WLAN) access. To better facilitate thevarious radio access technologies, UE 103 is a multi-radio terminal(MRT) that is equipped with an LTE radio transceiver 104 and an ISM(Industrial, Scientific and Medical) radio transceiver 105 co-located inthe same device platform (i.e., In-Device).

Due to radio spectrum regulation, different radio access technologiesmay operate in overlapping or adjacent radio spectrums. For example, thetransmission of the LTE radio transceiver can interfere with thereception of the ISM radio transceiver. Likewise, the transmission ofthe ISM radio transceiver can interfere with the reception of the LTEradio transceiver. How to effectively mitigate coexistence interferenceis a challenging problem for co-located radio transceivers operating inoverlapping or adjacent frequency channels. The problem is more severearound the 2.4 GHz ISM radio frequency band. The 2.4 GHz ISM band (e.g.,ranges from 2400-2483.5 MHz) is used by both fourteen WiFi channels andseventy-nine Bluetooth channels. In addition to the crowded ISM band,3GPP LTE Band 40 (e.g., TDD) ranges from 2300-2400 MHz, LTE Band 41(e.g., TDD) ranges from 2496-2690 MHz, and LTE Band 7 (e.g., UL FDD)ranges from 2500-2570 MHz, all are very close to the 2.4 GHz ISM radiofrequency band.

In one novel aspect, a method of implicit indication for unusable/usablefrequency is provided such that upon detection and indication ofin-device coexistence (IDC) interference, effective frequency divisionmultiplexing (FDM) solutions may be applied for IDC interferenceavoidance. In step 1, the LTE radio module of UE 103 detects IDCinterference from the co-located ISM radio module and triggers thereport of IDC problem (e.g., via a central control entity 110 forcontrol info). More specifically, the LTE radio module detects that theserving frequency becomes unusable due to IDC interference. In step 2,the LTE radio module of UE 103 reports the detected IDC interferenceproblem to its serving eNB 101. The reporting message, without thecontent explicitly associated with the serving frequency index or itsfrequency location, implicitly indicates to eNB 101 that the servingfrequency becomes unusable. In step 3, based on the reporting message,eNB 101 determines that an unusable frequency region between the servingfrequency and the ISM band. eNB 101 will treat all the frequencies inthe unusable frequency region be unusable when making IDC interferenceavoidance decisions.

FIG. 2 is a simplified block diagram of a base station eNB 201 and auser equipment UE 202 in accordance with one novel aspect. UE 202comprises memory 212, a processor 213 having a central control entity214, an LTE/LTE-A transceiver 224, a GPS receiver 223, a WiFitransceiver 222, a Bluetooth transceiver 221, and bus 225. Similarly,eNB 201 comprises memory 241, a processor 242, a control entity 243, andan LTE/LTE-A transceiver 244. The different entities are functionmodules that can be implemented by software, firmware, hardware, or anycombination thereof. The function modules, when executed by theprocessors 211 and 241 (e.g., via executing program codes 212 and 245),allow UE 202 and eNB 201 to perform various functions accordingly.

In the example of FIG. 2, central control entity 214 is a logical entityphysically implemented within processor 213, which is also used fordevice application processing for UE 202. Central control entity 214 isconnected to various transceivers within UE 202, and communicates withthe various transceivers via bus 225. For example, WiFi transceiver 222transmits WiFi signal information and/or WiFi traffic and schedulinginformation to central control entity 214 (e.g., depicted by a thickdotted line 231). Based on the received WiFi information, centralcontrol entity 214 determines control information and transmits thecontrol information to LTE/LTE-A transceiver 224 (e.g., depicted by athick dotted line 232). In one embodiment, LTE transceiver 224 detectsIDC interference problem and indicates such problem to its serving basestation eNB 201 (e.g., depicted by a thick dotted line 233). Based onthe IDC problem indication, eNB 201 makes certain decisions (e.g.,handover) to mitigate the IDC interference (e.g., depicted by a thickdotted line 234). Although the central control entity is implementedwithin processor 213 in the example of FIG. 2, it can be implementedwithin other modules such as the LTE transceiver, or be implemented asan independent and separate function module.

FIG. 3 illustrates an ISM radio signal and implicitly indicatingunusable serving frequency in accordance with one novel aspect. For IDCinterference, when the transmit signal by a WiFi/BT transceiver is veryclose to the receive signal for the co-located LTE transceiver, theWiFi/BT signal is the aggressor and the LTE signal is the victim. On theother hand, when the transmit signal by the LTE transceiver is veryclose to the receive signal for the co-located WiFi/BT transceiver, theLTE signal is the aggressor and the WiFi/BT signal is the victim. As ageneral observation, the coexistence interference from the aggressor tothe victim in adjacent frequency channels is generally decreasing whenthe frequency separation is enlarged.

In the example of FIG. 3, the WiFi/BT radio signal in the ISM band isthe aggressor and the LTE signal in LTE Band 40 is the victim. The WiFisignal has a first power amplitude of 2.922 dB at central frequencylocation 2.412 GHz (e.g., at marker 1), a second power amplitude of−49.33 dB at frequency location 2.37 GHz (e.g., at marker 2), and athird power amplitude of −50.33 dB at frequency location 2.32 dB (e.g.,at marker 3). Because the WiFi signal is much stronger at its centralfrequency location and significantly decreases as the frequencydecreases (or increases in the other direction), the coexistenceinterference to the LTE signal would also decrease when the LTE servingfrequency is farther away from the WiFi central frequency. Similarly,the coexistence interference to the LTE signal would increase when theLTE serving frequency is closer to the WiFi central frequency. As aresult, if a UE has an LTE serving frequency, and the UE detects thatthe serving frequency becomes unusable due to coexistence interferencefrom the ISM band, then the UE is able to determine that any otherfrequency channels located between the serving frequency and the ISMband would also be unusable because the coexistence interference wouldbe stronger as compared to the serving frequency. The frequency regionbetween the LTE serving frequency and the ISM band is referred to as“implied unusable frequency region”. Based on this specificcharacteristic of IDC interference, the UE can more efficiently reportIDC interference problem via implicit signaling.

FIG. 4 illustrates a method of implicit signaling design for IDCinterference avoidance in accordance with one novel aspect. Assume thatthe condition to judge unusable frequency is in principle similar as thecondition to trigger UE reporting of coexistence problem. When a UEdetects that its LTE serving frequency is unusable due to IDCinterference from the ISM band traffic, the UE reports the detected IDCinterference problem to its serving eNB. The IDC interference problemmay be reported explicitly or implicitly. For explicit reporting, the UEexplicitly identify a list of frequencies that are unusable. It is astraightforward reporting method but has high reporting overhead.

In one advantageous aspect, the UE applies implicit reporting method.First, the UE reports an indication of a detected coexistence problem tothe eNB without explicit content to describe whether the servingfrequency is unusable. The eNB, however, will treat the servingfrequency (e.g., at marker 1 in FIG. 4) unusable after receiving UEreporting of coexistence interference problem. Second, the UE reports anindication of the detected coexistence problem to the eNB withoutexplicit content to describe whether the frequency channels between theserving frequency and the ISM band are unusable. The eNB, however, willtreat the frequency region (e.g., region 401 in FIG. 4) between theserving frequency and the ISM band unusable after receiving UE reportingof coexistence interference problem. This implicit reporting methodsaves signaling overhead. The indication may be transmitted by sending adummy Channel Quality Indicator (CQI) value, dummy Reference SignalReceived Quality (RSRQ) measurement report, or by a Radio ResourceControl (RRC) message in UL control channel. For example, the UE mayreport the lowest CQI or the lowest RSRQ to implicitly indicate the IDCinterference problem. In an extension of the implicit reporting method,the UE only reports the boundary of the implied unusable frequencyregion, and the eNB treat all the frequency channels between thatboundary and the ISM band unusable.

In the example of FIG. 4, the UE also reports the indication of thedetected coexistence problem to the eNB with another embedded indicationto indicate whether there is unusable or usable frequency channeloutside the implied unusable frequency region 401 between the servingfrequency and ISM band. For example, the UE may use one single bit toindicate whether there is usable frequency channel outside the impliedunusable frequency region. If there is no usable frequency channel inthe entire LTE Band 40, then the eNB may have no choice other than toactivate TDM-based coexistence avoidance mechanism. On the other hand,if there is usable frequency in outside the implied unusable frequencyregion, then the eNB may consider applying FDM-based coexistenceavoidance mechanism.

In one example, the eNB may select some of the frequencies (e.g., atmarker 2 and marker 3) outside the implied unusable frequency region,and then request the UE to perform measurements over those selectedfrequencies. The eNB can then determine suitable frequency for handoveroperation to mitigate the coexistence interference. For instance, if themeasurement result indicates that the frequency channel at marker 3 isusable, then the eNB can determine an implied usable frequency region402 that is farther away from the ISM band than the frequency channel atmarker 3. As a result, the eNB can select any frequency channel withinthe implied usable frequency region 402 for handover operation tomitigate IDC interference. In another example, the eNB is more likely toselect a usable frequency channel that is farther away from the ISM bandfor handover operation. For instance, the eNB starts from the frequencychannel that is farthest away from the ISM band, and selects frequencychannel for handover operation as long as available.

In addition to implicitly indicating to eNB that a UE serving frequencyis unusable, the UE may also send an indication to eNB when the servingfrequency becomes usable. The indication may be transmitted by sending adummy CQI value, a dummy RSRQ measurement report, or by a RRC messagevia UL control channel. For example, the UE may report the highest CQIor the highest RSRQ to implicitly indicate the relief from IDCinterference problem. First, the UE indicates that the frequencychannels between the serving frequency and the ISM band become usable bysending an indication of the relief of coexistence problem to the eNBwithout the content explicitly associated with the serving frequencyindex or frequency location. The eNB, however, will treat the frequencyregion between the serving frequency and the ISM band as usable inimplicit manner after receiving the IDC relief indication. Second, theUE indicates that the serving frequency becomes usable by sending anindication of the relief of coexistence problem to the eNB without thecontent explicitly associated with the serving frequency index orfrequency location. The eNB, however, will treat the serving frequencyas usable in implicit manner after receiving the IDC relief indication.The eNB will also treat the frequency channels that are farther awayfrom the ISM band than the serving frequency as usable frequencychannels. Furthermore, the UE can indicate if there is unusablefrequency between the serving frequency and the ISM band by anindication not explicitly associated with specific frequency index orfrequency location. Based on this further indication or eNB judgment, ifthere is unusable frequency (e.g., indicated by one single bit), thenthe eNB can request the UE to perform measurements over at least some ofthe frequency channels between the serving frequency and the ISM band.

How to configure UE measurement pattern is another important aspect inhandling IDC interference problems. UE measurement is not only fornormal measurement reporting to eNB, but also is closely related to howto judge whether a frequency is usable/unusable, as well as when totrigger the reporting of IDC interference problems. With multipleco-located radio modules on the same device platform, it becomes moreimportant for the UE to perform measurements at the right time. Forexample, if the LTE radio signal is interfered by the ISM radio signal,then the UE should perform measurement during the period when ISM isexpected to be actively transmitting so that the measurement result ismore meaningful in terms of more accurately reflecting the IDCinterference problem. In LTE systems, because UE measurement patternsneed to be configured by the network, it is thus helpful if the networkhas some knowledge of the ISM traffic information before configuring anymeasurement pattern for UE.

FIG. 5 illustrates a method of configuring a measurement pattern in anLTE system 500 in accordance with one novel aspect. LTE system 500comprises a UE 501 and an eNB 502. UE 501 is equipped with an LTE radiomodule and a co-located ISM radio module. In step 511, UE 501 reportsits ISM traffic information to eNB 502. In step 512, eNB 502 determinesa measurement pattern at the period where ISM is expected to be activelytransmitting based on the reported ISM traffic information. In oneexample, eNB 502 could customize a measurement pattern based on thereported ISM traffic info. In another example, eNB 502 could select oneof the pre-defined measurement patterns based on the reported ISMtraffic info. The selected pattern could be the one that leads to mosttime overlap between the measurement gap and the ISM TX period. In step513, eNB 502 sends the determined measurement configuration to UE 501.In step 514, UE 501 sends measurement result to eNB 502. UE 501 may alsotrigger the reporting of coexistence interference. UE 501 may implicitlyindicate unusable serving frequency and unusable frequency region to eNB502. Finally, in step 515, eNB 502 makes handover decision to mitigateIDC interference. The above illustrated measurement configurationmechanism may not always work due to signaling overhead. Differentalternatives may be considered.

FIG. 6 illustrates a first alternative of measurement configuration inan LTE system 600 in accordance with one novel aspect. LTE system 600comprises a UE 601 and an eNB 602. In step 611, eNB 602 configures aprohibit timer to UE 601 so that UE 601 refrains from making unnecessaryIDC problem reporting. Note that the eNB may configure this timer to UEvery early, e.g., when UE just enters its coverage. The configurationmay be done by UE specific message or broadcast message. In step 612, UE601 performs measurements of LTE radio signals and triggers IDC problemreporting. Note that UE 601 performs the measurements without beingconfigured with a specific measurement pattern based on the real ISMtraffic information. In step 613, UE 601 sends a first IDC indication toeNB 602. The IDC indication may implicitly indicates unusable servingfrequency and unusable frequency region to eNB 602 due to coexistenceinterference. At the same time, UE 601 also starts the prohibit timer.In step 614, eNB 602 make certain decisions to mitigate the coexistenceinterference problem based on the received IDC indication. Before theprohibit timer expires, UE 601 is refrained from sending a second IDCindication to eNB 602. In step 615, after the prohibit timer expiration,UE 601 sends a second IDC indication to eNB 602 if UE 601 stillexperiences coexistence interference.

The benefit of this method is that the measurement detail is up to UEimplementation. Because of signaling overhead, eNB 602 has no idea ofthe exact measurement pattern, so eNB 602 can configure a timer toprevent unnecessary IDC reporting to protect the network from frequentand inaccurate IDC reporting from the UE. In addition to the prohibittimer mechanism, other criteria associated with system performance maybe used as a condition to limit UE measurement implementation. In afirst example, the UE cannot indicate IDC problem if a Physical DownlinkControl Channel (PDCCH) decoding rate is above a threshold value. In asecond example, the UE cannot indicate IDC problem if a downlink HARQerror rate is below a threshold value. In a third example, the conditionis satisfied if the eNB informs the UE that the eNB is capable ofsupporting in-device coexistence interference avoidance feature. Forexample, if the eNB configures a prohibit timer to be infinite, it mayimply that the eNB does not support IDC feature at all. So thisconfiguration actually means whether eNB can support IDC feature or not.

Another alternative is that the eNB simply configures a set ofmeasurement periods for IDC measurement. The benefit of this method ishaving a simplified measurement period configuration regardless of thereal ISM traffic. The measurement configuration may be determined by L3filtering parameters. If the measurement report is to reuse or modifycurrent RRM measurement report procedure, it is good to consider thefiltering parameters. If eNB already has some pre-knowledge of the IDCoperation (e.g., beacon periodicity, WiFi offload from CN), it ispossible that eNB properly configures a moderate measurement period.

If an LTE UE is also equipped with a Global Navigation Satellite System(GNSS) radio for GPS service, the GNSS reception may be interfered bythe co-located LTE transmission. In one advantageous aspect, the UEobtains TDM coexistence interference avoidance information from itsserving eNB. For example, the eNB informs the UE when it may schedule UEto transmit uplink signal. The UE then informs its GNSS receiver thetime period that the UE may transmit uplink signal. Based on thisinformation, the GNSS receiver may choose to drop the decoding resultsover the timer period when the in-device LTE transceiver may transmituplink signal. This is because the decoding result over the interferedsignals may not be accurate and would cause errors on the positioningresults. This time period may be based on the TDM traffic patterninformation obtained from the eNB. Alternatively, this time period maybe based on real-time UE internal signaling on the LTE transmissionstatus.

FIG. 7 is a flow chart of a method of IDC interference avoidance fromeNB perspective. In step 701, an eNB receives an IDC interferenceindication from a UE having an LTE radio module and a co-located ISMradio module. The indication indicates that a serving frequency becomesunusable due to a coexistence interference problem. The indication doesnot explicitly indicate a frequency index or a frequency location of theunusable serving frequency. In step 702, the eNB determines an impliedunusable frequency region based on the received IDC indication. Theimplied unusable frequency region is between the serving frequency andthe ISM band. In step 703, the eNB configures a condition for the UE,such that the UE is refrained from sending IDC interference indicationsunless the condition is satisfied. Note that this step 703 isindependent from the previous two steps 701-702. Typically, the eNBconfigures this condition before the UE possibly make any IDCinterference indication.

FIG. 8 is a flow chart of a method of IDC interference avoidance from UEperspective. In step 801, a UE measures a received radio signal using anLTE radio module that is co-located with an ISM radio module. In step802, the UE sends an IDC interference indication to an eNB. Theindication indicates that a serving frequency becomes unusable due to acoexistence interference problem. The indication does not explicitlyindicate a frequency index or a frequency location of the unusableserving frequency. In step 803, the UE sends an IDC interference reliefindication to the eNB. The relief indication indicates that the servingfrequency becomes usable. The relief indication does not explicitlyindicate a frequency index or a frequency location of the usable servingfrequency.

Although the present invention has been described in connection withcertain specific embodiments for instructional purposes, the presentinvention is not limited thereto. For example, although an LTE-advancedmobile communication system is exemplified to describe the presentinvention, the present invention can be similarly applied to othermobile communication systems, such as Time Division Synchronous CodeDivision Multiple Access (TD-SCDMA) systems. Accordingly, variousmodifications, adaptations, and combinations of various features of thedescribed embodiments can be practiced without departing from the scopeof the invention as set forth in the claims.

What is claimed is:
 1. A method comprising: receiving an in-devicecoexistence (IDC) interference indication from a user equipment (UE)having an LTE radio module and a co-located Industrial, Scientific andMedical (ISM) radio module, wherein the IDC interference indicationindicates a coexistence problem of a serving frequency; configuring aprohibit timer that prohibits the UE from reporting two consecutive IDCinterference indications within a timer period; determining an impliedunusable frequency region by a base station, wherein the impliedunusable frequency region is between the serving frequency and ISM band;receiving an indication of whether there is usable frequency outside theimplied unusable frequency region; and selecting a usable frequencychannel for UE handover if the indication indicates possible usablefrequency, wherein the usable frequency channel is located in LTE bandexcluding the unusable frequency region.
 2. The method of claim 1,wherein the base station determines the implied unusable frequencyregion without requesting further measurement report from the UE.
 3. Themethod of claim 1, wherein the base station is more likely to select theusable frequency channel that is farther away from the ISM band.
 4. Themethod of claim 1, further comprising: receiving an IDC interferencerelief indication from the UE, wherein the relief indication indicatesthe serving frequency becomes usable, and wherein the relief indicationdoes not explicitly contain a frequency index or a frequency location.5. The method of claim 4, wherein the relief indication implicitlyindicates that frequency channels farther away from the ISM band thanthe serving frequency are usable frequency channels.
 6. The method ofclaim 1, further comprising: requesting the UE to perform furthermeasurements over a frequency channel that is outside the impliedunusable frequency region if the indication indicates possible usablefrequency.